Functional anatomy of the frontal aslant tract and surgical perspectives

Abstract

The frontal aslant tract (FAT) is an intralobar white matter fasciculus providing dense connections between the medial part of the superior frontal gyrus, in particular the pre-supplementary motor area (SMA) and the SMA proper, and the lateral part of the frontal lobe, especially the inferior frontal gyrus. Although this tract has been characterized belatedly, it has received important attention in recent years due notably to its increasingly evidenced role in the speech and language networks. As cerebral tumors frequently affect the frontal lobe, an improved knowledge of the functional anatomy of the FAT is mandatory to refine the way neurosurgeries are performed and to give the patients the best opportunities to recover after surgery. In this work, we first describe the spatial arrangement of the FAT and detail its cortical projections. We then provide a comprehensive review of the functions supposedly mediated by this transverse frontal connectivity. It is structured following a tripartite organization where the linguistic (i.e. speech and language), supralinguistic (i.e. functions that interact with speech and language: executive functions, working memory, and social communication) and extralinguistic implications (i.e. functions outside the linguistic domain: visuospatial processing, praxis and motor skills) are successively addressed. We lastly discussed this knowledge in the context of wide-awake neurosurgeries for brain tumors. We emphasize the need to evaluate thoroughly the functions conveyed by FAT by means of longitudinally-designed studies to first estimate its plasticity potential and then to determine which tasks should be selected to avoid lasting impairments due to its disconnective breakdown.

Segregated circuits for phonemic and semantic fluency: A novel patient-tailored disconnection study

“…Disconnection analyses revealed that lower scores in phonemic fluency were related to left FAT and IFOF partial damage/disconnection, specifically in its frontal part. The left FAT plays a pivotal role in speech production and initiation ( Chernoff et al, 2019 , Dragoy et al, 2020 , Kemerdere et al, 2016 , Kinoshita et al, 2015 ), and implications in working memory and executive functions (in particular, inhibition) have been reported (see Burkhardt et al, 2021 , for a review). Interestingly, these last cognitive processes are also involved in lexical retrieval on phonemic cue ( Robinson et al, 2012 , Shao et al, 2014 , Troyer et al, 1997 , Unsworth et al, 2011 ).…”

“…The involvement of the FAT in verbal fluency on phonemic cue is in line with previous results ( Catani et al, 2013 ) on neurodegenerative diseases. Moreover, superior cortical terminations of this tract are found in the Supplementary Motor Area (SMA and preSMA) ( Burkhardt et al, 2021 , Catani et al, 2013 ): a lesion in this part of the superior frontal gyrus, anterior to the precentral gyrus, may lead to the so-called SMA syndrome, implying transient akinesia, mutism and dysexecutive deficits ( Bannur and Rajshekhar, 2000 , Sjöberg et al, 2019 , Vergani et al, 2014 ). At the same time, the SMA is connected by the FAT to the inferior frontal gyrus, in particular to Broca’s area ( pars opercularis and pars triangularis ) ( Burkhardt et al, 2021 , Catani et al, 2013 ), which is involved in speech articulation and phonemic fluency ( Birn et al, 2010 , Cattaneo et al, 2011 , Heim et al, 2008 , Sarubbo et al, 2020 , Zacà et al, 2018 ).…”

“… Superior longitudinal fasciculus, third branch (SLF III): inclusion ROIs for the segmentation of the SLF III ( Petrides and Pandya, 1984 ) were placed on the coronal plane in the IFG and within the SMG, excluding fibers running towards the temporal lobe, as this bundle connects the SMG with the IFG ( De Benedictis et al, 2014 , Forkel et al, 2014 , Howells et al, 2020 , Kamali et al, 2014 , Martino et al, 2013 , Vavassori et al, 2021 , Zacà et al, 2018 ). Frontal aslant tract (FAT): the FAT is an oblique WM tract that connects the superior frontal gyrus (SFG, in particular SMA/pre-SMA) to the IFG (mainly pars opercularis and pars triangularis ) ( Burkhardt et al, 2021 , Catani et al, 2013 , Catani et al, 2012 , Kinoshita et al, 2012 ). The FAT was dissected using two inclusion ROIs: the first one was placed on the axial plane within the SFG, and the second one on the sagittal plane, just over the IFG.…”

“…Frontal aslant tract (FAT): the FAT is an oblique WM tract that connects the superior frontal gyrus (SFG, in particular SMA/pre-SMA) to the IFG (mainly pars opercularis and pars triangularis ) ( Burkhardt et al, 2021 , Catani et al, 2013 , Catani et al, 2012 , Kinoshita et al, 2012 ). The FAT was dissected using two inclusion ROIs: the first one was placed on the axial plane within the SFG, and the second one on the sagittal plane, just over the IFG.…”

The unique role of the frontal aslant tract in speech and language processing

“…One recently described tract is the frontal aslant tract (FAT), which runs from the supplementary motor area to the pars opercularis of the inferior frontal gyrus (IFGOp) ( Catani et al, 2013 , Thiebaut de Schotten et al, 2012 ). Since its initial description, research into the structure and function of the FAT has greatly expanded the current understanding of the tract, contributing not only to the neurocognitive understanding of the tract but also to its consideration in a clinical context ( Burkhardt et al, 2021 , La Corte et al, 2021 ). One area that the FAT has been commonly associated with includes both motor speech disorders (e.g.…”